Readers of this blog are well aware of autotomy in lizards – self-amputation of the tail – that usually occurs as a result of sub-lethal predation. Readers of this blog are also familiar with the fascinating ability of many lizards to regenerate new tails post-autotomy. Lizards are the closest relatives to humans that can regenerate a fully functional appendage in the adult stage, and understanding the molecular basis of this process can shed light on the latent regenerative capacities in mammals. A new paper published this week in PLOS ONE (Hutchins et al. 2014) provides the first insights into the genetic mechanisms of lizard tail regeneration, using Anolis carolinensis as a model. Via the high-throughput sequencing of RNA from regenerating green anole tails, and the mapping of these sequences to the A. carolinensis genome, the authors describe the genes that are expressed during the regeneration process, shedding light on potential targets for future human therapies.
Disclaimer: I am not an author on the paper, although I do work in the Kusumi Lab with the authors.
While the ability to regenerate a fully functional appendage in the adult phase is likely a deeply homologous trait across animals, it is not uniformly conserved across vertebrates. Fish, as in the zebrafish model (Gemberling et al. 2013), and amphibians, as in the salamander models (Knapp et al. 2013) can regenerate both limbs and tails, suggesting that while the ancestral vertebrate was equipped with this ability, it seems mammals have during their evolution somehow lost it. Evolutionary hypotheses explaining exactly why some taxa lose the ability to regenerate adult appendages are far and wide, ranging from the stochastic to ecologically-specific fitness trade-offs (reviewed in Bely and Nyberg 2010).
But what are the proximate (i.e. genetic) reasons as to why lizards remain strong regenerators while mammals are left holding the short end of the regeneration stick? Sharing an amniote common ancestor that excludes fish and amphibians, lizards and humans pretty much share the same genes. Perhaps rather than requiring the acquisition of entirely new sets of genes or gene families, the ability to regenerate involves modifications of existing genetic pathways and spatiotemporal patterning of regulatory counterparts. In a nutshell: it may be that the genetic toolbox for regeneration had “shut off” during the evolution of mammals while remaining intact in lizards. Since the regenerated lizard tail contains new cartilage, muscle and nervous tissue, we would like to know precisely which genetic pathways are used to “turn on” the regeneration of those tissue types. This knowledge would have real potential to make better genetic therapies available to people who have experienced catastrophic injuries.
To understand the molecular bases of tail regeneration in lizards, Hutchins et al. (2014) used RNA-Seq data from five distinct sections along the proximal-distal axis of regenerated lizard tails and analyzed the differential expression of genes in each region. They identified 326 differentially expressed genes in total – and 302 of these have clear mammalian orthologs. In the regenerated tail base, gene ontology (GO) groups were associated predominantly with skeletal muscle. Meanwhile, genes that were differentially expressed at the tip were enriched for GO categories related to (i) wound response, (ii) hormonal regulation and (iii) embryonic morphogenesis. Differentially expressed genes in the Wnt signaling pathway in a proximal-distal gradient along the regenerating lizard tail axis suggest similarity to the regenerating salamander tail tip and mouse digit tip. This conserved role of Wnt and other pathways among tetrapod vertebrates suggests that the aforementioned yet previously unknown genetic toolbox for regeneration in amniotes is shared by all tetrapods, and may have particular relevance for translation into human medical approaches.
A lot of internet media has picked up on the new research, including an article in the Huffington Post, and Wired.com has picked a figure from the PLOS ONE article depicting a series of stained cross sections of regenerating anole tails from 10 to 25 days post-autotomy as their Science Graphic of the Week. A link to the paper is fast-approaching 4,000 upvotes on Reddit as I write this blog post. Unfortunately, The Daily Beast featured a spot on the research, but used a photo of the wrong lizard. In my opinion, the article on IFLScience.com was the most honest and accurate. Not surprisingly, certain press outfits will tend to overplay, or at least mischaracterize, the potential benefits to human regenerative medicine. One intrepid reporter wonders if this research will bring humans closer to having super-healing powers like the Wolverine.
Laughable, indeed, yet linking lizard’s regenerative abilities to comic book characters has certainly been done before, most notably during the Silver Age of comics when Stan Lee and Steve Ditko created one of the greatest villain origin stories of all time (the current author’s herpetological proclivities notwithstanding). In Amazing Spider-Man #6 (1963), scientist and war amputee Dr. Curt Connors, obsessed with finding a way to regrow his arm, injects himself with a lizard-derived serum. The experiment works initially, and his arm regenerates (after he spends a night passed out on the lab floor, naturally). But things go awry, he transforms into The Lizard, and he’s been foiled by that blasted web slinger ever since.
Of course, we are not doing these kinds of experiments in the Kusumi Lab. Besides, the regenerated lizard tail is actually very different from the original, consisting of an unsegmented cartilaginous rod surrounded by muscular bundles that lack symmetry. This would not be very functional if you were interested in regrowing the well-organized segments of limb muscle and bone that arose during embryonic development. Basically, the precise structure that the lizard regenerates is not really what a human would want. Hutchins et al. (2014) report the expression of genes in the lizard regenerated tail that promote chondrogenesis, neurogenesis, and myogenesis. All of these processes represent tissues that have high interest for human health, and present some of the first real evidence that the Anolis genome can provide useful medical insights.